1. Field of the Invention
The present invention relates to the improvements of a reciprocating piston of an internal combustion engine suitable for automotive vehicles.
2. Description of the Prior Art
In reciprocating pistons used for automotive internal combustion engines, during reciprocating motion of the piston, the piston serves to transmit combustion pressure through a piston pin and a connecting rod to a crank pin and thus convert the combustion pressure into rotational force (torque) of an engine crankshaft. The piston operates with the piston crown or piston head exposed to extremely hot combustion gases, whereas the piston skirt contacts the comparatively cool cylinder wall. This results in a temperature gradient from the top of the piston to the bottom. Generally, the temperature of the piston top exposed to the combustion chamber is higher than that of the piston bottom. Thus, there is a difference of thermal expansion from the top to the bottom. Additionally, the piston moves up and down at high speeds, during engine operation. Of various engine parts, the piston is always subjected to very severe circumstances, namely thermal stresses and mechanical stresses. The piston must have satisfactory durability to live under these severe conditions, while performing its function and while smoothly sliding against the cylinder wall. During the operation of the engine, the resultant force, which is obtained as the product of the combustion pressure applied to the piston crown and the inertia force of the reciprocating engine parts, acts on the piston. With the connecting rod tilted, the resultant force is divided into a component force acting in a direction of the connecting rod, and a component force (called side thrust or major thrust) acting in a thrust direction perpendicular to the direction of action of the resultant force. In order to dispersedly transmit the side thrust acting on the cylinder wall or the cylinder bore, the piston is formed with a piston skirt at both sides of piston pin-boss portions. The greater the circumferential width of the piston skirt, the greater the contact-surface area (or the thrust face) of the piston skirt. With the greater thrust face of the skirt, the side thrust can be effectively dispersed, thus avoiding high localized stresses acting on the cylinder wall. However, the greater the skirt surface area or the circumferential width of the skirt, the greater friction loss during the reciprocating motion of the piston, thus increasing power loss of an internal combustion engine. To balance these two contradictory requirements, that is, increased friction loss and good dispersion of side thrust, there have been proposed and developed various unsymmetrical piston skirt structures where two sides (a major thrust side and a minor thrust side) of the piston skirt are unsymmetrical with respect to the piston pin-boss portions. Such a light-weight piston having an unsymmetrical skirt, has been disclosed in Japanese Utility-model Provisional Publication No. 64-3054 and in U.S. Pat. No. 4,274,372. In the conventional piston structures with an unsymmetrical skirt, as disclosed in the Japanese Utility-model Provisional Publication No. 64-3054 and in the U.S. Pat. No. 4,274,372, a skirt surface area of a major thrust side, on which a comparatively great side thrust acts when the piston descends during the power stroke, is dimensioned to be greater than a skirt surface area of a minor thrust side, on which a comparatively small side thrust acts when the piston moves upwards during the compression stroke, so as to effectively disperse the side thrust force, while, at the same time, reducing friction loss. As is generally known, the differences in thermal expansion between the cylinder and piston during operation, caused by variations in temperature, would change the fit between the cylinder wall and the piston skirt such that it would be either loose to tight. If the fit is too tight, high contact-surface pressure may occur between the cylinder wall and piston skirt owing to thermal expansion, thereby resulting in wear. To reduce undesired cylinder-wall wear or skirt wear and to satisfy various requirements, namely increased flexibility of the skirt in the thrust direction for thermal-expansion control, proper durability (to such an extent that permanent set never takes place under great side thrust), and less possibility of deformity by thermal or mechanical causes, the previously-noted piston structure with an unsymmetrical skirt is often utilized. Referring now to FIG. 11, there is shown a bottom view of a prior art piston 10 with an unsymmetrical skirt, disclosed in the Japanese Utility-model Provisional Publication No. 64-3054. As seen in FIG. 11, the major-thrust-side skirt 20 has a greater circumferential width than a minor-thrust-side skirt 22. The piston 10 is formed integral with two diametrically opposing connecting wall portions (24, 24), each interconnecting one side edge of the major-thrust-side skirt 20 and one side edge of the minor-thrust-side skirt 22 via the associated piston pin-boss portion 18. Each of the connecting wall portions (24, 24) is formed into a substantially circular-arc shape so that each connecting wall portion expands radially outwards. The radius-of-curvature R of each of the connecting wall portions (24, 24) is dimensioned to be greater than the radius of the piston 10. Additionally, the center-of-curvature C of each of the connecting wall portions (24, 24) is offset somewhat to the major thrust side (see the eccentricity E shown in FIG. 11). With the connecting wall portions 24 and 24, each expanding radially outwards, as a whole, the rigidity of the piston (in the radial direction) can be lowered. Therefore, even when the piston experiences interference fit between its side wall and the cylinder wall, the side wall of the piston is able to effectively deflect by virtue of proper flexibility of each connecting wall portion (24, 24), thereby avoiding the contact-surface pressure between the cylinder wall and the piston skirt surface from excessively rising. This reduces undesired cylinder-wall wear or skirt wear. In FIG. 11, reference sign 14 denotes a piston crown portion, whereas reference sign 26 denotes a stiffening rib portion. However, with the previously-noted piston skirt structure, the angle at intersection point between the minor-thrust-side skirt 22 and each of the connecting wall portions (24, 24) is an obtuse angle. Furthermore, in comparison with the major-thrust-side skirt 20, the circumferential width of the minor-thrust-side skirt 22 is short. Thus, the rigidity of the minor-thrust-side skirt 22 remains kept high. On the other hand, both the radially-outward expanded connecting wall portions 24 and 24 contribute to reduction in the radial durability of the piston. Actually, the piston pin-boss portions (18, 18) are located in the middle portion of the respective connecting wall portions (24, 24). Each of the pin-boss portions has a comparatively high rigidity. Probably, it will be impossible to induce adequate deflection of the connecting wall portions.
Frictional resistance imposed on the piston is broadly classified into (i) a frictional force created between the cylinder wall and the major-thrust-side skirt surface on expansion or power stroke, caused by a relatively great thrust force occurring owing to the combustion pressure, and (ii) a frictional force created between bearing surfaces of the cylinder wall and piston during the intake, compression, and exhaust stroke and caused by inertial force of the reciprocating parts and thermal expansion with less effect of combustion pressure or without providing the effect of combustion pressure. Practically, the engine operation is greatly effected by the frictional resistance applied to the piston at comparatively low engine speeds, and thus the magnitude of thrust force arising from inertia force based the reciprocating motion of the piston is negligibly small, as compared to the magnitude of thrust force occurring on the power stroke. The greater part of the frictional resistance imposed on the piston during the intake, compression, and exhaust stroke can be regarded as a frictional force created between bearing surfaces of the cylinder wall and piston owing to thermal expansion. Through various studies and searches, the inventors of the present invention have analyzed that the sliding resistance of the piston occurs due to frictional resistance between bearing surfaces of the cylinder wall and piston skirt, and additionally the frictional resistance or frictional force can be considered to be equivalent to shearing stresses or shearing force existed in lubricating oil undergoing viscous shear and prevailing between the cylinder wall and the piston skirt, when side thrust force acts in the thrust direction perpendicular to the piston-pin direction. In order to effectively reduce the frictional resistance, it is desirable to provide a means for reducing a normal component of the reaction of the pressure-receiving sliding surface of the piston side wall (or the piston skirt surface), in other words a side thrust force, and also for reducing the surface area of the pressure-receiving sliding surface of the piston skirt for reduced coefficient of friction.
Accordingly, it is an object of the invention to provide a piston of an internal combustion engine which avoids the aforementioned disadvantages of the prior art.
It is another object of the invention to provide a piston of an internal combustion engine, which is capable of reducing friction forces during four strokes of the engine, improving fuel economy, and enhancing engine performance, by reducing a coefficient of friction between the cylinder wall and piston, (that is to say, reduced pressure-receiving sliding surface area of the piston skirt) from the viewpoint of a frictional resistance (or a comparatively large side thrust) imposed on the major thrust side on the power stroke, and by reducing a frictional force created between bearing surfaces of the cylinder wall and the piston skirt owing to thermal expansion from the viewpoint of a frictional resistance imposed on the major thrust side and on the minor thrust side on the intake, compression, and exhaust stroke.
In order to accomplish the aforementioned and other objects of the present invention, a piston of an internal combustion engine comprises a piston crown portion, a pair of piston pin-boss portions, each integrally formed with the piston crown portion and having a piston-pin hole, a piston skirt adapted to be in sliding-contact with a cylinder wall and having a major-thrust-side skirt portion and a minor-thrust-side skirt portion, and a plurality of web-like apron portions, each interconnecting a side edge of either one of the major-thrust-side skirt portion and the minor-thrust-side skirt portion and either one of the pair of piston pin-boss portions, wherein a projected circumferential width of the minor-thrust-side skirt portion is greater than a projected circumferential width of the major-thrust-side skirt portion, and a minimum thickness of the minor-thrust-side skirt portion is less than a minimum thickness of the major-thrust-side skirt portion. It is preferable that the minor-thrust-side skirt portion is dimensioned to satisfy an inequality W/Dxe2x89xa730.8xc3x97(T/D)+0.15, where W denotes the projected circumferential width of the minor-thrust-side skirt portion, D denotes a cylinder bore, and T denotes the minimum thickness of the minor-thrust-side skirt portion. Preferably, each of the major-thrust-side skirt portion and the minor-thrust-side skirt portion may comprise a pair of stiffening rib portions integrally formed on an inside wall thereof and being continuous with associated web-like apron portions of the plurality of web-like apron portions. The pair of stiffening rib portions are spaced from each other by a predetermined projected circumferential width in a direction parallel to an axial line of the piston-pin hole. It is preferable that the pair of stiffening rib portions of the minor-thrust-side skirt portion are located at a lower level than the pair of stiffening rib portions of the major-thrust-side skirt portion with respect to the axial line of the piston-pin hole, and the predetermined circumferential width between the pair of stiffening rib portions of the minor-thrust-side skirt portion is greater than the predetermined circumferential width between the pair of stiffening rib portions of the major-thrust-side skirt portion. Preferably, each of the pair of stiffening rib portions of the major-thrust-side skirt portion and the pair of stiffening rib portions of the minor-thrust-side skirt portion may have a substantially trapezoidal shape in cross section taken in an axial direction of the piston, and a thickness of each of the pair of stiffening rib portions of the major-thrust-side skirt portion and the pair of stiffening rib portions of the minor-thrust-side skirt portion is gradually decreased towards an innermost end thereof in a circumferential direction of the piston. The piston may further comprise a slit which is formed in the back of a lowermost piston ring groove of the plurality of piston ring grooves and located in a side of the piston corresponding to the minor-thrust-side skirt portion so that the slit penetrates a side wall of the piston.